Contents

These planets are little bigger than dots in the night sky, but if you call them dots, then Earth (in the marble-sized group, topleft) is smaller than a grain of sand.

Wikijunior books welcomes you to the children's book Solar System. Outer space is perhaps the final frontier for humanity. Even though the rest of the solar system objects may seem like tiny dots from Earth, our celestial neighbors are still important to learn about. If, when you grow up, you are going to be an astronaut and travel in space, you will need to know quite a bit about the solar system. And even if you don't travel to space, the things other people do there will affect you, so you need to know about it. Also, if you meet an astronomer or an astronaut, you do not want to sound ignorant! The importance of learning about the solar system has led many experts here at Wikijunior to donate their time and talents to bring this volume together.

Wikibooks is a project of the Wikimedia Foundation, aimed at providing free, easily available, quality reading for adults and children to promote the global spread of knowledge. Traditional publishing houses make the bulk of their income from re-issues of classic books, new books by authors with long track records, or celebrities who are famous in their own right. The chances of a truly good new work being published solely on the basis of merit skyrocket when the traditional business model is overturned and the wellspring of new talent out there is tapped using the Internet.

With this project we have reached a crossroads between the books of yesterday and the encyclopedia of everything for tomorrow. Simply by reading this book and telling others about it, you have advanced the cause of free access to information and of democratizing the field of publishing. Thank you, and once again, welcome.

A stamp issued by the Soviet Union, showing Sputnik's planned orbit path around the Earth.

Scientists are still exploring the universe. Whether things are very tiny, like the cells of plants and animals, or very big like a solar system or a galaxy, there is still a lot scientists don’t know.

Scientists who study space are called Astronomers or Astrophysicists. They explore the solar system in two different ways. Astronomers do it by observing celestial bodies through telescopes, while astrophysicists (a specialized class of astronomers) try to explain the observed phenomena using physics, as suggested by the name, and theorize about what is still unseen or unknown.

Telescopes were invented in the early 1600s in Europe and allowed curious scientists like Galileo Galilei to look at very distant things in close-up and see details of the solar system and the universe that nobody had ever seen before. Using his telescope, Galileo was the first person to see the rings around Saturn, and draw a very detailed picture of the moon. He also saw the four largest moons of Jupiter, sometimes called the Galilean moons, and saw spots on the Sun. Telescopes on Earth and in space are still used to explore the Solar System. There are several types of telescopes. The most common ones are optical telescopes, such as Galileo's (optical means relating to light, which what these telescopes see), and radio telescopes, that pick up radio waves from outer space (radio waves occur naturally; they don't have to be made by humans).

Until the 1950s, humans were limited to exploring the Solar System from the ground. However, in 1957 the Soviet Union (now Russia and several other countries) launched the very first satellite, Sputnik 1 (pronounced like spoo-tneek). Since then, humans have been launching vehicles into space to explore the Solar System—some manned (with people) and some unmanned (without people).

Now, the Solar System is full of human-made probes, exploring the planets and moons of the solar system. The probes send back information to Earth that scientists study to figure out what it means. Every year, scientists are learning more about the Solar System. Sometimes they learn things about other worlds that remind us of Earth. Other times, the things they learn are very strange. Everything they learn helps us to understand more about Earth, Earth's history and Earth's neighborhood.

It’s important that scientists use measurement to tell how big, how hot or cold, or how far away something is. In science, people use the metric system, which is named after its basic unit the metre. Below is a description of all the types of measurement used in this book.

For indication of measurements, how distant something is, or how long or wide it is, scientists use kilometres or metres. Units of the metric system (a kilometre is 1000 metres, a metre is little more than 3 feet in the old imperial unit system still in use in some regions). Kilometres is often shortened to km and metres is often shortened to m. Kilometres and metres can also be spelled as Kilometers and Meters, but the International Bureau of Weights and Measurements uses the -re versions as the official spelling.

Since distances outside of the Earth get so vast, scientists have also invented new units of measurement to make it easier to measure large distances in space. They invented the Astronomical unit (㍳) which is equivalent to 149 597 871 kilometres. One Astronomical Unit is the approximate distance between the Earth and the Sun. The mean distance between the sun and Neptune (the farthest planet from the sun) is 30.1 ㍳, or 4.503 billion kilometres. This is why it's good to use ㍳ for a big distance like that: 30 distances from the Earth to the sun is easier to understand than four and a half billion kilometres. You might not realize there was something wrong if someone told you the distance from the sun to Neptune was 45.03 million kilometres, but if you thought of it as 0.301 ㍳, you would know it couldn't be right.

In astronomy, they have leveling-up scales similar to the metric (10 mm in 1 cm, 100 cm in 1 m) and customary (12 in. in 1 ft., 3 ft. in 1 yd.) scales. Usually these distances are not used within the solar system, but they are important to know if you want to be an astronomer or astrophysicist.

1 Light-year (ly)=63241.077 ㍳

1 Parsec (pc)=3.26 ly

1 Kiloparsec (kpc)=1000 pc

1 Megaparsec (mpc)=1000 kpc

1 Gigaparsec (gpc)=1000 mpc

To help you visualise just how big some of these are,

4.22 ly=The distance from Earth to the nearest star (Proxima Centauri) other than the sun

1.3 pc=The distance from Earth to Proxima Centauri

34 kpc=The length of the Milky Way

0.76 mpc=The distance from Earth to the nearest Galaxy, the Andromeda Galaxy

In order to measure how big something is, scientists measure the mass of an object in kilograms or grams. There are 1000 grams in a kilogram. Scientists do not use weight, because weight is a measurement that means how hard gravity is pulling on an object. An object’s mass is the same wherever you are in the solar system because it measures how much stuff, or matter, a thing is made up of. Your weight will change because the amount of gravity there is varies from place to place.

On Earth, mass and weight are the same. If you weigh 30kg (short for kilograms) on Earth, your mass is 30kg. If you are floating around in space, your weight if you try to stand on a set of scales will be 0kg, but your mass is still 30kg. You are still made up of the same amount of matter.

Temperature is a numeric reference, in a scale of degrees, on how hot or cold something is in relation to a "constant" reference. There are several scales. In our everyday lives we measure temperature in degrees Celsius, written °C for short (the little circle ° means "degrees"); or degrees Fahrenheit, written °F for short. But scientists, especially astronomers, use degrees Kelvin to measure temperature, written K for short (with no °). Don't use Celsius or Fahrenheit for temperatures in astronomy!

Some important temperatures to know in Kelvin:

0K (−273.15°C) is maximum coldness, also called absolute zero. This is a great thing about measuring temperature in Kelvin, that the number is always positive, and tells you how much warmer things are than the coldest they could possibly be.

The freezing point of water is 273.15K (0°C), and the boiling point of water is 373.15K (100°C).

A sunny day of 30°C, 86°F, would be 303.15K. This is 273.15 + 30, because degrees change equally in Kelvin and Celsius.

The Solar System is a Wikijunior book written by a group of volunteers and made freely available to Internet users, printers, and distributors under the terms of its license. It is the result of cooperation between The Beck Foundation, The Wikimedia Foundation, and volunteer writers and editors.

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